A simple fix here ... DO NOT cut any corners til you sew!
Take the 2-1/2" square and draw a diagonal line across the square.
Set the square on top of the corner of your larger square that will become the snowball.
Stitch on the line.
Then after you cut the excess off, leaving 1/4" for the seam.
If the small square has set squarely on the large square as you sewed, once you flip the piece over and press it, you should have a square corner.
Continue on for all four corners!
Good Luck!! ...